Processes for embossing and for debossing and perforating heat-softened thermoplastic film are known. As used herein, a web of material is a thin, flat, substantially planar web of material of definite, preferably substantially uniform, width and indefinite length. A web of embossed film or other material is such a web of such material in which numerous protuberances have been formed to provide it with a substantial third dimensional structure perpendicular to the plane of the material. A web of debossed and perforated film or other material has protuberances which provide it a third dimensional structure perpendicular to the plane of the material in which many or all of the protuberances have holes. The protuberances of webs of embossed and of debossed and perforated materials may be of a wide variety of sizes and shapes or may all be identical. Collectively such webs of embossed and/or debossed and perforated materials are referred to herein as webs of formed materials; the protuberances of such webs of formed films provide the webs with a three-dimensional bulk conformation.
In prior art processes for producing a web of formed film, a web of heat-softened film is provided on a patterned, perforated outer surface (referred to herein as a forming surface) of a structure such as an endless belt or a drum cylindrical surface. A vacuum beneath the forming surface pulls the heat-softened film into conformity with the forming surface. Alternatively, a positive pressure may be used to force the heat-softened film against the forming surface. Whether the web of film is simply embossed or is debossed and perforated will depend on the size of the holes in the forming surface, the softness and thickness of the film being formed, and the fluid pressure differential across the film.
Processes for producing webs of embossed thermoplastic film are disclosed in U.S. Pat. Nos. Re 23,910 issued to Smith & Smith on Dec. 12, 1954; 2,776,451 and 2,776,452 both issued to Chavannes on Jan. 8, 1957; and 2,905,969 issued to Gilbert & Prendergast on Sept. 29, 1959. Processes for the production of webs of debossed and perforated thermoplastic films are disclosed in U.S. Pat. Nos. 3,038,198 issued to Shaar on June 12, 1962; 3,054,148 issued to Zimmerli on Sept. 18, 1962; 4,151,240 issued to Lucas & Van Coney on Apr. 24, 1979; 4,155,693 issued to Raley on May 22, 1979; 4,226,828 issued to Hall on Oct. 7, 1980; 4,259,286 issued to Lewis, Sorensen & Ballard on Mar. 31, 1981; 4,280,978 issued to Dannheim & McNaboe on July 28, 1981; 4,317,792 issued to Raley & Adams on Mar. 2, 1982; 4,342,314 issued to Radel & Thompson on Aug. 3, 1982; and 4,395,215 issued to Bishop on July 26, 1983. A process for the production of perforated seamless tubular film is disclosed in U.S. Pat. No. 4,303,609 issued to Hureau, Hureau & Gaillard on Dec. 1, 1981.
The processes disclosed in the references cited above require that the thermoplastic film be heat-softened in order to achieve the desired embossing or debossing and perforation of the film. This can be achieved as disclosed in many of the above references by heating an existing web of film to a temperature above its melt temperature range such that it is in a molten state and will readily flow and attain a new configuration. Alternatively, the molten film may be achieved by feeding a web of film direcly from a film extruder onto the forming surface. Such a process is disclosed in U.S. Pat. No. 3,685,930 issued to Davis & Elliot on Aug. 22, 1972, where a web of thermoplastic film is extruded directly onto the outer surface of an endless belt and a vacuum is pulled beneath the belt to make the molten web of film assume the configuration of the outer belt surface. Similarly, U.S. Pat. No. 3,709,647 isseud to Barnhart on Jan. 9, 1973, discloses a web of molten thermoplastic film extruded directly onto the outer cylindrical surface of a vacuum forming drum.
It is known to shape molten thermoplastic sheet material by the use of a fluid pressure forcing the sheet against a mold; such processes are disclosed in U.S. Pat. Nos. 2,123,552 issued to Helwig on July 12, 1938; and 3,084,389 issued to Doyle on Apr. 9, 1963.
The production of a foraminous material by deposition of liquid latex on the surface of a rotating cylinder is disclosed in U.S. Pat. Nos. 3,605,191 and 4,262,049 both issued to Kasper on Sept. 20, 1971, and Apr. 14, 1981, respectively. Holes in the latex film may be achieved by rupturing with high pressure water jets.
When webs of embossed or debossed and perforated thermoplastic film are produced on a patterned surface by the above prior art processes, it is generally necessary to cool the film below its melt temperature range to set its three-dimensional structure prior to removing the web of formed film from the forming surface. This makes the web of formed film much less susceptible to distortion of its bulk conformation.
This need to cool and set a web of formed film prior to its removal from the forming surface is often a rate limiting step in the production of such formed film. A substantial amount of cooling of webs of formed film can occur by heat transfer from the film to the forming surface. In Davis & Elliott, such heat transfer is enhanced by cooling the endless belt after stripping the web of formed film from the belt and before a web of molten film is again applied to the belt. In Barnhart, a cooling medium is used inside the drum to cool the forming surface.
Cooling can also be assisted by contacting the outer film surface with an external cooling medium while the web of formed film is on the forming surface. For webs of debossed and perforated film, cooling of the formed film is usually assisted by drawing air through the perforations with vacuum prior to removing the web of formed film from the forming surface; such a cooling process is disclosed, for example, in Lucas & Van Coney. Chavannes '452 discloses contacting a web of embossed film with a wet belt to cool the film. Gilbert & Prendergast discloses use of a water spray to cool a web of embossed film on the forming surface of an embossing drum. Doyle discloses cooling vacuum formed molten thermoplastic sheet with a water mist.
A disadvantage of making webs of formed film by these prior art processes is the need to have the film within or above its melt temperature range in order to form the film. This limits the range of desired properties that can be engineered into the formed film since all previous thermo-mechanical history of the film is erased. Also, the rapid cooling of the formed film necessary for retention of the desired bulk conformation of the film produces a very limited set of molecular morphologies and, hence, limited range of physical properties in the resultant formed film.